That is a good question and I think he is overly excited and just completely over reacts and puts the other dogs on the defensive.....which seems to escalate the situation. I admit that it scares me and I'm sure that he senses my anxiety which makes it even worse. Clearly I need help learning how to deal with this. Hopefully the training I get from this will prove invaluable!!
Try and see if you can tell next time you're out, it may also help knowing what might be behind the reaction.
